Diarrhea is a common condition characterized by increased frequency or fluidity of bowel movements. Diarrhea may cause dehydration and electrolyte abnormalities that may require hospitalization to replace lost fluids and electrolytes until the symptoms subside. Persistent, uncontrolled diarrhea can cause such severe malnutrition, electrolyte imbalances and dehydration that it may ultimately result in death. Acute diarrhea is usually treated with fluid and electrolyte replacement, dietary modifications and antidiarrheal or antimicrobial agents. Acute diarrhea complications may cause severe illness, especially in high-risk groups, for example patients with underlying immunosuppression or advanced age. Antidiarrheal treatment is also required in patients with chronic diarrhea. Empiric therapies routinely used for chronic diarrhea include: stool-modifying agents (such as psyllium and fiber), anticholinergic agents, opiates, antibiotics, and probiotics.
Chronic diarrhea may be a symptom of a chronic disease, for example irritable bowel syndrome (IBS). It has been estimated that the prevalence of chronic diarrhea in the United States is approximately 5%. IBS alone is estimated to affect 15-20% of the U.S. population, and accounts for at least 30% of all gastroenterology health care costs. In many cases, the cause of the chronic diarrhea is not found, the diagnosis remains uncertain, and empiric treatments unsuccessful. Thus, there is an ongoing need for antidiarrheal agents that effectively stop or greatly reduce bowel movements and fluid loss in patients undergoing treatment, to remove the cause of diarrhea, or in patients in which the cause of diarrhea is not found.
H1 and H2 receptor antagonists are two classes of antihistamines. H1 receptor antagonists are used in the symptomatic treatment of multiple conditions, including allergic rhinoconjunctivitis, relief of pruritus in patients with urticaria, and in patients with chronic asthma. Newer H1 receptor antagonists, such as cetirizine, are referred to as second-generation H1 receptor antagonists, and are more selective for peripheral H1 receptors than first-generation H1 receptor antagonist, which antagonize both the central and peripheral nervous system H1 receptors as well as cholinergic receptors. The selectivity significantly reduces the occurrence of adverse drug reactions, such as sedation, while still providing effective relief of allergic conditions.
H2 receptor antagonists are used primarily to treat symptoms of acid reflux, or gastroesophageal reflux disease. H2 receptor antagonists reduce the production of stomach acid. Often diarrhea is listed as a major side effect of H2 receptor antagonists.
Diphenhydramine, a first-generation H1 receptor antagonist, together with either cimetidine or ranitidine, H2 receptor antagonists, have been studied for the treatment of acute allergic reactions. In a first study (Runge et al. “Histamine antagonists in the treatment of acute allergic reactions” Ann Emerg Med (March 1992) 21:237-242), patients were treated by a single intravenous administration of a solution 300 mg cimetidine and placebo, 50 mg diphenhydramine and placebo, or diphenhydramine plus cimetidine; the treatment was found effective for acute urticaria. In a second study (Lin et al. “Improved outcomes in patients with acute allergic syndromes who are treated with combined H1 and H2 antagonists” Ann Emerg Med (November 2000) 36:462-468), patients were treated by a single parenteral administration of a solution of either 50 mg diphenhydramine and saline or 50 mg diphenhydramine and 50 mg ranitidine; the treatment was found effective for acute allergic syndromes.